The present invention relates to a method for the starting and steady-state supply of a permanent-magnet synchronous motor, particularly for driving a hydraulic pump of the centrifugal type.
Permanent-magnet synchronous motors are highly advantageous, because they are highly efficient in operation.
However, the problem that arises particularly for high-power motors driving loads which have a considerable inertia is the starting step.
Mechanical means or electronic control procedures have been adopted in order to solve this problem according to the prior art.
The mechanical means consist in particular in uncoupling the impellers of the driven hydraulic pumps, so that the rotor can start freely through an angle of less than 360.degree. and then engage the impeller and continue its rotation.
It is evident that this solution can be used when the rotor has a low mechanical inertia which in any case allows it to reach the steady state in a half-period if, as normally occurs, the rotor has two poles.
In practice, this means that it is not possible to use these mechanical devices in order to have a medium- to high-power pump which involves rotors whose dimensions and consequent inertia are a function of said power.
Electronic starting procedures are also known in which the rotor is controlled in its position and the sinusoidal mains current is "chopped" with static switches so that it is in a phase which does not contrast the movement of the rotor.
This solution, which uses the current without modifying the mains frequency, does not allow to achieve high static torques and therefore still does not allow to supply medium- to high-power motors.
Other methods use an inverter which generates a waveform whose frequency gradually rises, the behavior of this frequency being stored beforehand in the power supply circuit.
This solution, too, suffers considerable problems, since permanent-magnet electric motors are all different from each other, particularly as regards the magnetic characteristics of the rotor, despite being manufactured with identical dimensions.
Motors of this type are also different as regards the stator pack.
At the stator level, the problem is modest, since there can be only differences due to the lamination pack and to the windings of the coil.
However, these differences are not particularly relevant and important in terms of influence on the magnetic circuit.
The greatest differences instead occur in the rotors, because they are not identical and practically never have an equal and uniform direction of the north and south fields.
There is always a north and a south, but with respect to the geometry of the rotor the field shapes are usually the ones designated by 10 and 11 in FIG. 2 with respect to a cylindrical permanent-magnet rotor, designated by the reference numeral 12.
This entails first of all an asymmetry in the movement of the rotor, so that the motor runs unevenly.
A typical motor with permanent-magnet rotor is generally designated by the reference numeral 13 in FIG. 1, where the rotor is now designated by the reference numeral 14 and is contained between two poles made of laminations 15 and 16 which are the ends of a stator pack 17 whereon two coils 18 and 19 are mounted which induce the stator field.
In order to be able to privilege one direction of rotation for the rotor, the poles have recesses 20 and 21 which set the axis of symmetry 22 of the rotor 14 at an angle with respect to the median axis of the poles 15 and 16.
There is also a position sensor 23 which is arranged in the intermediate region between the poles 15 and 16.
This conventional structure suffers a further problem due to the displacement of the neutral axis of the rotor with respect to the position of the sensor 23.
All these problems prevent exact knowledge of the magnetic structure of the motor and of its true characteristics.
Another problem arises from the fact that the intended starting method is linked to the combination of a permanent-magnet synchronous motor and a centrifugal hydraulic pump.
The mechanical-hydraulic characteristics of the pump also determine problems in starting, since they too are neither known nor exactly predictable.
Many factors are in fact involved in the starting of a hydraulic pump.
For example, if the pump has been motionless for a long time, there may be problems in terms of deposits of products conveyed by the water which produce unknown initial loads.
This problem can also occur during rotation, when conveyed objects become located in the impeller chamber, sometimes to the point of jamming and locking the rotation of the pump.